Automatic heterodyne eliminator



.Sept 5, 1949- J. A. MCLAUGHL'IN AUTOMATIC HETERODYNE ELIMINATOR Filed Aug. 5, 1946 Kulm.

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Patented Sept. 6, 1949 UNITED STATES PATENT OFFICE AUTGMATIC HETERODYNE ELIMINATOR James L. A. McLaughlin, La Jolla, Calif.

Application August 5, 1946, Serial No. 688,403

(Cl. Z50-20) 14 Claims. l

This application relates to a heterodyne eliminator, 'and more particularly to an apparatus adapted to automatically eliminate an undesired signal so closely adjacent a desired signal that it lies in one of the sidebands associated with the desired signal and causes highly objectionable heterodyne interference.

One feature of this invention is that it provides improved apparatus for eliminating undesired signals and heterodyne interference caused thereby; another feature of this invention is that it provides automatic means for changing the frequency characteristics of an undesired signal without changing the frequency characteristics of the desired signal; yet another feature is that either of the two sidebands associated with a desired signal is rapidly and automatically selected, the other sideband being attenuated; a further feature of this invention is that it provides radio receiving apparatus able to automatically cut through and get a desired signal in the face of interference otherwise rendering such a signal unintelligible; and still another feature of this invention is that it provides an improved indication of correct tuning to a desired signal; other features and advantages of this invention will be apparent from the following specification and the drawings, in which:

Figure l is primarily a schematic block diagram of apparatus including a preferred embodiment of my invention with the machanical construction o-f a yportion of one feature of my invention shown in detail; Figure 2 is a schematic diagram of part of the apparatus shown in Figure 1 and included in my invention; and Figure 3 is a voltage versus frequency curve of a frequency discriminator such as is used in my invention.

A radio signal normally comprises three cornponent parts, a carrier wave of fixed frequency and two sidebands of varying width created by modulation of the carrier wave with the intelligence which it is desired to transmit. Receiving apparatus can, ofcourse, be designed with almost any desired amount of selectivity, but making the circuits too selective results in chopping olf the outer edges of the sidebands and in losing part of the intelligence. Accordingly, the usual radio receiver is provided with a selectivity of about ten kilocycles width, designed lto receive the carrier and two sidebands extending out to about 5,000 cycles on each side thereof. An even wider reception band is desirable for high fidelity reproduction; and anything narrower than about music, but even tends to make speech unintelligible.

It will be understood that the above discussion is directed to the type of amplitude modulation now generally used conventionally, and my invention Will be described in connection with such a system. It will be understood, however, that my invention will also find use with very narrow band radio telegraphy signals, or very wide band frequency modulated signals. The principles are the same in each case, and it is thought that it would unduly complicate the disclosure of my invention if an effort were to be made to describe it in connection with every possible system of transmittion and reception.

When an undesired signal carrier wave lies within a sideband, therefore, the ordinary radio receiving system is unable to eliminate it and it heterodynes with the desired signal carrier to produce an annoying single lfrequency tone which substantially or entirely Icrowds out the desired intelligence. For example, a pair of undesired signal carriers one and one-half kc. and two kc. lower in frequency than the desired carrier results in the production of three heterodyne beat notes of 500 cycles, 1500 cycles, and 2000 cycles per Second. If the undesired carriers are of substantial voltage amplitude the resultant beat notes will entirely .blanket out the desired intelligence.

As disclosed in my Patent No. 2,364,863, issued December 12, 1944 I have found that under such lconditions selection of the upper sideband and elimination of the lower sideband will entirely remove the undesired signals and their heterodyne interference without losing the intelligence, since it has long been known that all of the desired intelligence can -be derived from a single sideband, single sideband transmission and reception having -been used in certain specialized phases of radio work such as transoceanic radio telephony.

InV carrying out my present invention my preferred apparatus comprises switch actuated means whereby the receiver may be operated as a conventional receiver or as a single sideband receiver; and in the latter case the upper or the lower sideband is automatically selected dependingr upon which sideband contains no unwanted interference. I have found that such selection can be best accomplished yby using two oscillators,l

one a certain frequency above the frequency of the desired signal and the other an equal frequency below the frequency of the desired signal, and causing one or the other oscillator to heterodyne with the input which contains both the de- 6,000 cycles not only loses the higher notesn of sired and undesired signal, the selection of the oscillator being made automatically as a function of the undesired signal. In conjunction with this arrangement I employ a band pass filter cutting off sharply lbelow the changed frequency of the desired signal, and passing 4 or 5 kilocycles of frequencies above that of the changed desired signal, so that only one sideband is selected to furnish the desired intelligence.

The use of two such different heterodyning oscillators enables the elimination of two completely separate single band pass amplifiers, so that either sideband may lbe readily chosen with only a single band pass amplifier. It will be understood that this arrangement is applicable to single sideband transmitters as well as to receiverers, although it is here being disclosed only in connection with a receiver.

In the particular embodiment of my invention illustrated herewith it will be understood that the signal delivered to the intermediate frequency amplifier designated as Iii in Figure 1 is a desired signal at a conventional intermediate frequency. That is, my apparatus as disclosed in Figure 1 would be preceded by a conventional radio frequency amplifier, preferably tuned, a tuning oscillator and a mixer so that the desired radio frequency signal would be changed to a frequency intended for usual intermediate frequency use, as 455 kc. per second. Thus in a complete receiver embodying my invention there may be two stages of heterodyning through which the radio frequency signal passes before it is rectified to provide the audio-frequency intelligence, and I am terming the first intermediate frequency, which is the frequency delivered to the intermediate frequency amplifier designated at I in the particular embodiment Yof my invention shown in Figure l the desired signal, it being understood that this is lower than the original radio frequency signal, but higher than the second intermediate frequency.

By adjustment of the receiver tuning oscillator, therefore, the desired radio frequency signal has been converted to a frequency of 455 kc., this being the input to IF amplifier I0. This signal has two sidebands four or ve thousand cycles in width associated with it, and an undesired carrier frequency may have come through as a frequency of, for example, 452 kc., lying in the lower sideband.

I make use of two crystal oscillators, one adapted to oscillate precisely at a certain frequency above and the other adapted to oscillate at the same frequency below the frequency of the desired signal. In Figure 1 I have indicated one of these crystal oscillators as A, this oscillator operating at a certain frequency below the frequency of the desired signal, as 405 kc.; and the other as crystal oscillator B, this operating at the same frequency above the frequency of the desired signal, as 505 kc. Since both differ in frequency exactly 50 kc. from the frequency of the desired signal, heterodyning of the output of either of these oscillators with the desired signal in the mixer II will result in a changed desired signal of 50 kc. This changed signal will, of course, still have its sidebands with it. The undesired signal, however, can now be made to lie in either` sideband depending upon the oscillator used. If oscillator A is supplied to the mixer for heterodyning operation the undesired signal will have a frequency of 47 kc. in the case assumed, while if oscillator B is used as shown in Figure 1 the undesired signal will have a frequency of 53 kc.

The output of the mixer is divided into two portions, one being supplied to a single sideband pass amplifier I2 adapted to pass a frequency band from 50 to l54 kc., said amplifier being designed to cut off sharply below 50 kc. The output of this single sideband pass amplifier is supplied in a conventional manner to a detector I3 and then to an audio-frequency amplifier I4 to provide the desired audio output.

The general operation of the system heretofore described is in accordance with the system disclosed in my aforementioned Patent No. 2,364,863 and I have not illustrated the circuits of the various portions of a receiver operating in the manner just described since reference may be had to such patent to supplement the present disclosure in such regard.

My apparatus shown diagrammatically in Figure 1 would be preceded by the radio frequency amplifier, tuning oscillator and mixer of a conventional receiver, preferably of the communications type; my intermediate frequency amplifier I0 would be built in accordance with standard intermediate amplifier practice; my oscillators A and B would each in themselves be conventional fixed oscillators precision controlled to the desired frequency; the mixer II would, of course, follow conventional practice, as would also the single sideband pass ampliiier I2, the detector I3, and the audio frequency amplifier I4.

Whereas the apparatus utilizing switching disclosed in my prior Patent No. 2,364,863 depended upon manual switching means to select the correct oscillator to eliminate the heterodyne interference, my present invention makes such selection by means of a switch which is operated by the interference itself as a function of the conditions in the sidebands and which automatically selects the correct oscillator to eliminate the sideband containing the heterodyne interference. Such automatic heterodyne switching permits more rapid action, and in the presence of interference such as is so often encountered in time of war wherein the enemy attempts to block radio reception by rapidly shifting an interfering signal back and forth across the frequency of the desired signal, my new system will automatically switch the interference out as fast as it shifts from one sideband to the other.

In accordance with my present invention the other portion of the output of the mixer II is fed into another intermediate frequency amplier I5 which is tuned to the frequency of the changed desired signal, and this amplifiers output wave constitutes the input signal for a frequency discriminator I6 which is adapted to convert frequency difference into an actuating voltage. This voltage is applied to the input of a relay amplifier stage I'I, and results in a changed output current from said stage. The change in output current actuates a first or control relay I8, which in turn actuates a secon? or impulse relay I9, said impulse relay being v:(.daJed to select either oscillator A or oscillator 'f to heterodyne with the input signal as conditions may warrant, as will be hereafter explained.

Assuming that the desired signal of 455 kc. which comprises the input to IF amplifier I0 has two sidebands of 5 kc. each associated with it, and that the lower sideband contains an undesired signal of a frequency of 452 kc. (such frequencies of 455 kc. and 452 kc. having been ccnverted from higher radio frequencies in prior stages of the receiver), and that B oscillator is fixed at 505 kc. and is connected to the mixer Il, then the output 'of lmixer |2|Y will contain' the `desired signal `now changed to 50 kc..with its associated sidebands, the undesired jsignal. being now-included in the upper sideban'd as a frequency of 53 kc. A portion Vofthe-output of mixer lIl is delivered to the single sideband pass ampliiier I2 which is designed to pass and amplify frequencies :of from 50 to about 54.kc., but to cut off sharply below 50 fkc., and to taper ofi in its acceptance of frequencies above about 54 kc. The output of 'this amplier, now containing only one sideband, is supplied vin conventional manner to a Vdetector and then tolan-'audio frequency amplier to provide the-desired audio output, the operation of the -systemup to this point being similar to that described and shown in somewhat greater detail in'myaforementoned Patent`2,364,863.

My present invention lies in the novel use which ismade ofthe other portion ofthe output of -mixer I'I. Referring now to Figure 2, the tuned circuit 2U, including the condenser 20a and the inductance 26h, comprises the output circuit of a conventional intermediate frequency ampliiier designated generally as I5 in Figure 1. This amplier receives 'the other portion of the output of mixer II, and is tuned to pass 50 kc. and sidebands on each side thereof, the tuning of 4its output circuit being accomplished by varying the condenser a inthe particular circuit illustrated, `but it will be understood that such tuning and the tuning of other circuits illustrated in the drawings could also be accomplished by means of a variable inductance, or in any other conventional manner, and my invention should not be limited vto any particular Way of tuning the tunedrcircuits called for. The-output of thef50 kc. IF amplier I5 containing the changed desired signal of 50 kc. with its Yassociated sidebands, the upper sideband'including the changed undesired signal of 53kc., is ldelivered to afrequency discrimnator stage shown generally 'as I6 through both inductive and caf pacitive coupling, the latter being'through the condenser 20c, which may have a value of 100v micromicrofarad. Suchfrequency discriminator may be of any conventional type, the particular discriminator illustrated in Figure 2 lcomprising a tunedinput circuit I6a which is tuned "to 50 kc.; two diodes I6b, which may be contained in the same envelope, as in atype No. '6H6 tube; an outputrcircuit comprising the condenser IScand the .resistor Idvand 16e; and a lcircuit connect-` ing ithe electrical center of the output circuit to the electrical center of the input circuit, and including resistor 2|. The output ofthe frequency discriminator stage I6-comprises the in put of a relay amplifier stage I'I, the discriminator output being shown as directly coupled to the control `grid of the tube IIa vwhich may be a tube type No. 6V6 connected `as a triode. The now of plate current through tube 1a isY controlled by the variable resistor IIb connected between the cathode and ground, andis adjusted so that with no output from Vthe 'frequency discriminator, just enough plate current flows tooperate the control relay I8, which is connected in the Vplate circuit of the tube, so 'that with no output'from the'discriminator the `relayl contact I8a is pulled down to the position other thanthat shown in Figures 1 and 2. vkIn aparticular system 4-which I `*have built incorporating my invention, the control relay I8 is adjusted tooperate at vniillraxnperes and'to release at 23 mill-iamperes, nand the-cathode. resistor f Illav isfadjusted 'so 'that therelay just operates 'in the absence ofrany actuating voltage 'inthe output of thediscriminator. Therelay arm .I 8a.is spring biased to the vposition shown in the. drawings: when the `relay 'is released,'i. e., when the energizing current drops below 23ymi1liamperesn the examplegiven. Y

vA .frequency fdiscriminator operates fto *convert frequencies which varyfroma selected frequency, and which are :presented to its input, .into voltage variations in its output circuit. In the .particular circuit shown generally 'at I6 .in theldrawings, the .input vcircuit `I6a .is tuned to 50 kc. When va .frequency of .50 kc. `is presented to input circuit, the phase relationships are such that each half of the diode I6b is caused to conduct equally, :and the total voltage across Aresistors I'Gd and '|6e1in'the output circuitais zero. When .a vfrequency higher than 50 kc. is presentedto the input, the phase relationships arefsuch that -the diodes no longer conduct equally,land a negative voltage appears across resistors I6i and I6e in the output circuit, as shown graphically `in Figure `3; and when a frequency lower than 50 kc. is presented to the input, va positive voltage appears across resistors |6d and IBe in theoutput circuit.

In the case assumed above, two frequenciesarepresented to the input. 0f course, since the changed desired frequency of 50 kc.,is Ythe Vfrequency to which thediscriminator is tuned, no voltage change across the output resultstherefrom. However, the `changed undesired frequency oi 53 kc. results, Vas shown. in Figure 3,in anegative voltage appearing across resistors 16d and .I6e. This negative voltage is coupled tothe grid of tube .|1a, Aand causesa reduced :flow foi platefcurrent through this tube .in the conventional manner. When the lplate current is Vreduced, 'relay I8 is de-energized, -and the relay arm moves into the position shown inFigures .1 and 2. AAs willbe-seen from 'Figure 1, this'causes energization of the coi1122 of a second or-impulse relayv I9, Vand ythe arm. 23 Aof this relay is drawn down to a substantially vhorizontal position (when the relay is positioned as in Yliigure f1), further downward progress ofV the. arm beingprevented when the projection 24 strikes ythe core 25. The arm v23 Vis biasedV by the spring 23a to return to its upward orrelease position upon de-energiza-- tion ofthe coil 22 in the conventionalmanner, but contrary to conventional relay operation, when the arm returns to its 'releaseposition the relay contacts are 'not moved, the position of suchcontacts being changed only when `the relay a'rm 23 vmoves `from its release position to itsfoperating position upon energization of the coil. T his action is obtained by Vproviding two stationary contacts 26 and 21, and 'one contact 28 which is movable between two positions, land whichengages contact 26 `in one position, and engages contact 2'I in the other position. Move-- ment of lcontact 28 between its two positions is accomplished by means of a pawl 29 attached to relayv arm 23 and cooperating with a six toothed ratchet 3U which is rigidly mounted'together with a three sided cam 3| on a shaftl32. Contact :281is Imounted on an arm 33 of spring material, and is biased to normally engage contact 21, the'end-of the contact arm 33-engaging thecam 3|. Upon movement 'of 'the relay arm 23 from its release position as shown in the drawing to its operate position, the pawl 29 engages a tooth of the ratchet and moves the shaft holdingiithe ratchet fand the cam `through 4'one-sixth of a cycle, or 60. The end of the springlarm 33 which is shown as cooperating with the flat side of the cam so that the contact 28 engages the contact 2l in thedrawing now rides onto the adjoining apex of the cam, and the movable contact 28:.now engagesl contact 26. When the coil 22 is de-energized the pawl 29 slides over the cooperating tooth of: ratchet ,30, and the position of the contacts is not disturbed. vThe movable contact 28 is connected tothe input of the mixer, stationary contact 26; is connected to the output of A oscillator, and contact 21 is connected to the output of B oscillator, so that each time the impulse relay I9 is energized one or the other of the oscillators is selected to feed the mixer.

In the assumed case, when impulse relay'coil 22 becomes energized, contact 28` is moved by the operation of the above described mechanism to engage Contact 26, and B oscillator, being the 505 kc. oscillator, is switched out while A oscillator, being the 405 kc. oscillator, is switched into the operating circuit. Now the undesired signal of 452 kc. beats with 405 kc. and is changed to a frequency of 47 kc., and is eliminated by the single sideband pass amplifier I2, the opposite sdeband of 50-54 kc., which is free of the undesired signal, being delivered to the detector I3 and thence to the audio amplifier I4.

The other portion of the output of mixer stage II, also containing the changed undesired signal of 47 kc., is amplified by the IF amplifier I5 and presented to the frequency discriminator I6. Since this undesired signal is of lower frequency than the frequency to which the discriminator is tuned, a positive voltage appears at the discriminator output as shown by Figure 3. The positive voltage is applied to the grid of tube I'Ia, causing an increase in plate current through this tube, and consequently relay I8 is energized, and arm I8a pulls down, breaking the circuit to the impulse relay I9. The impulse relay is deenergized, and its arm 23 moves tothe position shown in the drawing, but the position of the contacts of such relay remains unchanged as heretofore explained.

`It will'be noted that when the frequency of the changed undesired signal is lower than the frequency of the changed desired signal at the input to the frequency discriminator, the plate current of the tube Ila is increased, causing relay i8 to operate, or to hold harder if it Was already operated.r yOn the other hand, if the frequency of the changed undesired signal is higher than the frequency of the changed desired signal at `the input to the frequency discriminator, the plate current of tube I la is decreased, causing relay I8 to release, and this in turn causes relay I9 to be energized with a consequent switching of oscillators.

Thus if B oscillator is switched in and an undesired signal initially of higher frequency than the desired signal, as 458 kc., appearsat the input of IF amplier I0, the desired frequency will again be changed to 50'kc., but the undesired frequency willV now be changed to 47 kc. This frequency vwill be blocked by the single sideband pass amplifier,v I2, which passes only 50 to 54 kc. The other portion of this signal which is delivered to the frequency discriminator causes'increased plate current through tube I'Ia, with the result that relay I8 is not released, but'is caused to hold harder, and consequently no shifting of oscillators occurs. f

. In the event that contact 28 engages contactl 26 when the set is first turned on so that A oscillator is switched in, an undesired signal higher than the desired signal at the 12F amplifier I0 will result in a switching'of oscillators, while an undesired signal lower than the desired signal will not result in a switching of oscillators, and in both cases theundesiredsignal will ultimately be changed to a frequency which is eliminated by the band pass amplifier I2.

Another advantage of my invention is that, even if .no undesired signals are present in the sidebands of the desired signal, a tuning indication is given the operator enabling him to obtain greater accuracy of tuning than is possible with a conventional radio receiver. It will be seen from Figure 3 that when the desired signal is tuned in correctly the frequency discriminator has an output of zero volts, and the relay amplifier tube I 'la is operating with normal plate current, holding relay I8 in operate position. If the desired signal is incorrectly tuned, the impulse relay I9 will automatically switch over to the other oscillator whenever the changed signal places a negative voltage on the grid ofthe relay amplifier tube Ila. Consequently, as the operator tunes into a desired signal, the system will automatically switch the sidebands as the tuning control crosses the correct setting, and a slight rocking of the tuning knob on both sides of the correct setting will result in a clicking sound as the relays operate and release, giving the operator an audible indication by which to obtain accurate tuning. The selectivity of tuning thus obtained can be broadened by increasing the current through the relay amplifier tube Ila by adjusting the variable resistor IIb.

An operative circuit embodying my invention might contain, in those portions here being irst disclosed, parts having the values and characteristics hereinafter immediatelyY stated. The control relay should be a sensitive relay, and may be of about 2500 ohms resistance and adjusted to operate at 25 milliamperes and to release at 23 milliamperes. The cathode resistor I'lb, with a tube of the type suggested, may be 5000 ohms; resistors IIid and IBe may be of 100,000 ohms each; and resistor 2I may be 75,000 ohms. The inductances 2Gb and IBa may be each 30 millihenrys,` and the condenser I6c 500 micromicrofarads,` for a discriminator tuned to 50 rkc. In one system which I have built a B+ voltage of 250 volts is used for the plate voltage of tube IIa. and to energize impulse relay I9. The restA of my `system may use parts of values in accordance with conventional receiver practice.

While I have shown and described a certain embodiment of my invention, it is to be understood that it iscapable of many modifications. Changestherefore, in the construction and arrangement rnay be ,made without departing from the spirit and'scope of the invention as disclosed in the appended claims.

`I claim: A j

1. Radio receiving apparatus of the character described adapted to eliminate an undesired signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; means for providing two separate waves, one having a frequency equal toA that of the desired signal plus a certain frequency-'and the other having a frequency equal to that of the desired signal less said certain frequency, whereby heterodyning of said waves with the desired signal changes the frequency of the desired signal A tothe same frequency in both cases while changing that of the undesired signal to different frequencies; and apparatus automatically -operable as a function of undesired signalconditions in l frequency in both cases While changing that of f the undesired signal to different frequencies; and

the sidebands for selecting one of said waves to 5 .sired signal of a predetermined frequency having converted sidebands; means for providing two separate waves of xed frequencies, onefhaving a frequency equal. to said predetermined frequency plusy a certain frequency and the other having a 'frequency equal to said predetermined frequency less said certain frequency, whereby heterodyning of said waves with the converted desired signal changes its frequency to thesame frequency in both cases while changing that of the converted undesired signal to different frequencies; and apparatus automatically operable` in response to said converted undesired signal for selecting one of .said waves to heterodyne with the converted desired signal rand for eliminating the changed undesired signal while retaining the changed desired signal.

3. Radio receiving apparatus of the character described adapted to eliminate an undesired signal closely adjacent a desired signal having upper and lower sidebands associated therewith, in-

cluding: means for providing a .changed desired i" signal of a predetermined frequency having changed sidebands; Va band passlter circuit adapted to pass one of said changed sidebands `and exclude the other and switch-actuated electrical apparatus' separate from said tunable means and automatically operable as a function of undesired sig-nal conditions in the sidebands,

for selectively reversing the .order `of said sidebands whereby one or thel lother 'of said sidebands lies within the pass band' of the filter cir-cuit. t y

Vll. Apparatus of the character described, including: means for providing a desired signal having sidebands associated therewith; an intermediate frequency band pass lter circuit adapted to pass one of said sidebands and exclude the other; means for providing two separate waves, one having a frequency equal to .that of the desired signal plus the intermediate frequency and the other having a frequency equal to that of the desired .signal less the intermediate -frequency; and automatic .electrical apparatus operable in response .to .the `presence of undesired energy inrthe .sidebands forselectively causing one or the other `of said waves 'to-heterodyne with' the desired signal to select which of said sidebands is to be passed by the filter circuit.

5. Radio receiving apparatus of the Icharacter described adapted to eliminate an undesired signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; two oscillators, one having a frequency equal to that of the desired signal plus a certain frequency and the other having a frequency equal to that of the desired signal less said certain frequency, whereby heterodyning of said oscillators with the desired signal changes electrical apparatus including a circuit automatically actuated as .a function of undesired signal conditions. in the sidebands f or selectively causing one or the other of said oscillators to heterodyne with the desired signal and for eliminating the changed undesired signal while retaining the changed desired signal.

6. Apparatus ofthe character described, including: means for providing a desired signal having sidebands associated therewith; andintermediate frequency band pass filter circuit adapted to pass one of said sidebands and exclude the other; twoy oscillators, one having afrequency equal to that of the desired signal plus the intermediate frequency and the other having a fref quency vequal to that of the desired signal less CTI the intermediate frequency;y and an automatic device operable in response to the presence of an undesired signal in the sidebands for selectively causing one or the other of said oscillators to heterodyne with the desired signal to select which of said sidebands is to be passed by the lter circuit.

7. Radio receiving apparatus of the character described adapted to eliminate an undesired signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; an intermediate frequency band pass filter circuit .adapted to pass one of said sidebandsand exclude the other; two oscillators, one having a frequency equal to that'of the desired signal plus the intermediate frequency and the other having a frequency equal to that of the desired signal less .the intermediate frequency; an electrical device automatically operable'as a .function of undesirable signal conditions in one of the sidebands for selectively causing oneV or the other of said oscillators to heterodyne with the desired signal to select which of said sidebands is to be passed by the filter circuit; and means for recti'fying the sideband passed by said lter circuit. v8. Radio receiving apparatus of the character 'described adapted to eliminate an undesired signal closely adjacent a `desired* signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; two oscillators, one having a frequency equalto that of the desired signal plus a certain frequency and the other having a frequency equal to that of the desired signal less said certain frequency, whereby heterodyning of said oscillators with the desired signal changes the frequency of the desired signal to the same frequency in both cases while changing that of the undesired signal :to different frequencies; a Vband pass' -lter circuit adapted to pass one Aof the sidebands associated with the changed desired signal and exclude the other; and automatic electrical apparatus operable in response to the presence of said undesired signal in one of said sidebands for selectively causing one or the other of said oscillators to heterodyne with the desired signal to select which of said sidebands is to be passed by the filter circuit.

9. Apparatus of the character claimed in claim 8 wherein said electrical apparatus includes impulse-operated switch means.

l0. Radio receiving apparatus of the character described adapted to eliminate an undesired signal 4closely adjacent a desired signal having sidethe frequency of the desired signal to the same It bands associated therewith, including: means for l providing a desired signal having sidebands associated therewith; means adapted to pass one of said sidebands and to exclude the other; means for providing two separate waves, one having a frequency equal to that of the desired signal plus a certain frequency and the other having a frequency equal to that of the desired signal less said certain frequency; and an automatic relay device operable in response to the presence of said undesired signal in one of said l sidebands for selectively causing one or the other Y of said waves to heterodyne with the desired signal to select which of said sidebands is to be passed.V

1l. Radio receiving apparatus of the character described adapted to eliminate an undesired signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; an intermediate frequency band pass lter circuit adapted to pass Yone of said sidebands and exclude the other; two oscillators, one having a frequency equal to that of the desired signal plus the intermediate frequency and the other having a frequency equal to that of the desired signal less the intermediate frequency; and relay apparatus responsive to the presence of said undesired signal in one of said sidebands for selectively causing one or the other of said oscillators to heterodyne with the desired signal to select which of said sidebands is to be passed by the filter circuit.

l2. Apparatus of the character described, in-

'.cluding: means for providing a desired signal having sidebands associated therewith; a band pass filter circuit adapted to pass one of said sidebands and exclude the other; means for providing two separate waves, one having a frequency equal to that of the desired signal plus a certain frequency and the other having a frequency equal to that of the desired signal less said certain frequency, whereby heterodyning of said waves with the desired signal changes the frequency of the desired signal to the same frequency in both cases while changing that of the undesired signal to kdifferent frequencies; apparatus for converting frequencies different from the frequency of the changed desired signal to an actuating voltage; and automatically operable apparatus controlled by such voltage, and l operable selectively to cause one or the other of said waves to heterodyne with the desired signal to select which of said sidebands is to be passed by the lter circuit. y

13. Radio receiving apparatus of the character described adapted to eliminate an undesired signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; an Yinte,rmediate frequency 12 band pass filter circuit adapted to pass one of said sidebands and exclude the other; means for providing two separate waves, one having a frequency equal to that of the desired signal plus the intermediate frequency and the other having a frequency equal to that of the desired signal less the intermediate frequency, whereby heterodyning of said waves with the desired signal changes the frequency of the desired signal to the same frequency in both cases while changing that of the undesired signal to different frequencies; a frequency discriminator for converting frequencies different from the frequency of the changed desired signal to an actuating voltage; a first relay connected to be actuated by such voltage; and a second relay connected to be operated by actuation of said rst relay, and operable selectively to cause one or the other of said waves to heterodyne with the desired signal to select which of said sidebands is to be passed by the filter circuit.

14. Radio receiving apparatus of the character described adapted to eliminate an undesired signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; an intermediate frequency band pass filter circuit adapted to pass one of said sidebands and exclude the other; two oscillators, one having a frequency equal to that of the desired signal plus the intermediate frequency and the other having a frequency equal to that of the desired signal less the intermediate frequency, whereby heterodyning of said oscillators with the desired signal changes the frequency of the desired signal to the same frequency in both cases while changing that of the undesired signal to different frequencies; a frequency Vdiscriminator for converting frequencies different from the frequency of the changed desired signal to an actuating voltage; a first relay controlled by such voltage; and a second relay connected to be operated by actuation of said rst relay, and operable selectively to cause one Y. of the other of said oscillators to heterodyne with the desired signal to select which of said sidebands is to be passed by the filter circuit.

JAMES L. A. MCLAUGHLIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,912,059 Barden May 30, 1933 2,050,679 Wheeler Aug. 11, 1936 2,255,645 Bligh et al. Sept. 9, 1941 2,293,274 Barber Aug. 18, 1942 2,364,863 McLaughlin Dec. 12, 1944 

